A. Field of the Invention
This invention relates to printing inks comprising a solid organic solvent for hot-melt ink-jet printers, and, more particularly, to such inks in which the solid organic solvent comprises one or more benzoate compounds.
B. Prior Art
Inks that are to be utilized in document printing by ink-jet application are subject to demanding and frequently conflicting requirements. Such inks are commonly used to print on opaque substrates, such as paper, in which case the printing must be sharp and clean for direct reading. Colored inks for such uses must contrast well with the substrate to form a distinct image.
Further, on a fibrous material (e.g. paper), the ink must not soak in and spread or "bleed" excessively, lest the sharpness of the image be unacceptably reduced. As an example of the degree of sharpness necessary, current applications require a resolution on the order of 240.times.400 (i.e. 240 dots per inch in the horizontal direction, and 400 dots per inch in the vertical direction) for text printing. High quality graphics printing may require resolutions extending up to 600 dots per inch. Also, the print must be resistant to abrasion, as well as to inadvertant transfer to adjacent materials. Additionally, since the document may be exposed to a variety of climates, the print should remain stable at temperatures ranging from -20.degree. C. to +70.degree. C. These restrictions are fairly severe when applied to black ink, but are even more severe when applied to colored inks, since even minor changes in color are frequently noticeable.
Different requirements are imposed if the ink must print on transparent materials for forming, for example, overhead projection transparencies. For projectable transparencies, the image must not only be sharp, but, in the case of colored inks, transparent as well. Such colored inks must produce an image which is highly transparent in order for the color quality to be retained when projected on a screen.
In the case of inks that are to be applied by hot-melt ink jet techniques, the problem is even further compounded by the necessity that the inks be compatible with jetting requirements of the printer. Typically, this application imposes restrictions on, for example, the viscosity of the inks, so that the inks can be expelled from the jet as relatively uniform droplets without excessive "satelliting", i.e. fragmentation of the ink droplet which results from improper jetting. Furthermore, the inks must be stable at operating temperatures as high as 150.degree. C. or even 175.degree. C. Additionally, the constituents must all be compatible with each other, both physically (i.e. capable of forming a solution which can be jetted successfully) and chemically (i.e. not adversely affecting the stability or other characteristics of the materials).
Many inks heretofore utilized in ink jet printers have been aqueous solutions of water-soluble dyes. Examples of such inks are described in U.S. Pat. No. 3,846,141, issued Nov. 5, 1974 to D. G. Ostergren et al, and U.S. Pat. No. 4,512,807 issued Apr. 23, 1985 to A. Ogawa et al. Inks of this type have been found acceptable for many applications, but do not produce the desired hardness in the resultant print, nor the requisite stability and sharpness for fine color applications.
An example of a non-aqueous ink jet ink is described in U.S. Pat. No. 4,390,369, issued June 28, 1983 to A. R. Merritt et al. The ink described therein utilizes a natural wax which is liquefied by heating in order to jet it and which is thereafter solidified on contact with the substrate, e.g. paper. Among other drawbacks, however, the resultant material readily abrades and is thus unacceptable for most document purposes where some degree of print stability is required.
Various benzoate compounds have been used in recording materials and printing compositions because of their desirable melting points and heat stability. U.S. Pat. No. 4,583,104, issued Apr. 15, 1986 to Iwakura et al. and U.S. Pat. No. 4,585,483, issued Apr. 29, 1986 to Ikeda et al., both describe the use of selected benzoates as "thermofusible substances" for use in heat sensitive recording materials. U.S. Pat. No. 4,426,227, issued Jan. 15, 1986 to Keeling et al. discloses a printing medium comprising a thermoplastic resin and a "rheology controlling diluent". The diluent is selected from a group of materials including "high molecular weight esters", among which are listed neopentyl glycol dibenzoate and triethylene glycol dibenzoate. U.S. Pat. No. 4,628,000, issued Dec. 9, 1986 to Talvalkar et al. describes a thermal-sensitive ink formulation for use in thermal printing comprising a mixture of sucrose benzoate and other components. None of these references describes or discusses any formulations for use as hot-melt ink-jet ink.